1. Field of the Invention
This invention relates to an apparatus for inducing a local flow in a fluid, such as for example, an impeller for aerating a liquid and an aeration method using the impeller. More particularly, this invention relates to an aeration method and an apparatus using an impeller having blades that flex as a function of impeller speed, so that at lower impeller speeds the blades are substantially radial, and at full operating speed the blades are fully backward-curved, thereby maintaining a more constant velocity of fluid exiting the impeller.
2. Description of Related Art
Radial impellers or rotors are used in many devices, such as vacuum cleaners, fluid pumps, air compressors, mixers and wastewater aerators. These devices typically comprise a circular central plate or hub, onto which a number of impeller blades are mounted. The performance of an impeller is a function of many variable factors, such as impeller diameter, blade width, blade angle and/or curvature, number and spacing of blades, etc. For a given impeller geometry, the impeller's performance will be most efficient at a specific impeller rotation speed (RPM), with specific inlet and outlet conditions. This optimum impeller speed and the specific inlet and outlet conditions are termed the impeller's "design conditions." The performance of an impeller typically is less than optimal when the impeller operates at other than its design conditions.
It has been found that non-radial, backward-curved impeller blades provide increased volumetric capacity, allowing an impeller to pump more fluid for a given power input than with radial blades. By "backward-curved," it is meant that a blade curves along its length, in a direction opposite the direction of the impeller's rotation, from the blade's root to its tip. Non-radial impeller blades, however, are generally more expensive to manufacture than are radial blades.
In many applications, it is important that the velocity of the fluid exiting the impeller (the "exit velocity" or "local flow velocity") be controlled. For example, in wastewater aeration applications, it is critical that the water leaving the impeller have a minimum local flow velocity, relative to the remainder of the body of wastewater, of approximately 12 feet per second (ft./sec.). At this speed, "white water" is generated, which is where aeration occurs. If the local flow velocity of the wastewater from the impeller drops below 12 ft./sec., sufficient aeration will not be achieved. It is therefore common for aeration facilities using known, fixed-blade impeller configurations to drive the impellers at a rotational speed which results in a local flow velocity incrementally greater than 12 ft./sec.--commonly approximately 18 ft./sec. Increasing the local flow velocity beyond the minimum needed to generate white water, however, does not appreciably improve aeration. Thus, driving the impeller at higher rotational speeds than necessary to achieve the minimum local flow velocity needed for aeration wastes energy, and unnecessarily increases operating costs.
Many aeration facilities attempt to reduce unnecessary energy consumption by controlling the rotational speed at which an aeration impeller is driven. For example, many aeration installations use two-speed electric motors to drive the aeration impellers. Alternatively, variable-frequency drives (VFD's) may be used with the drive motors for the impellers, enabling the motor speed to be continuously controlled within an operating range. Some installations use dissolved oxygen monitors to control the drive speed of the impellers responsive to the oxygen content of the wastewater. However, with known, rigid-blade impeller configurations, the local flow velocity of water from the impeller is a function of the speed of rotation of the impeller. Therefore, adjusting the rotational speed of the impeller's drive motor directly affects the local flow velocity of water discharged from the impeller. As discussed above, a local flow velocity which is too low results in insufficient aeration, whereas a local flow velocity which is too high wastes energy. Thus, known, rigid-blade aeration impeller configurations suffer the disadvantage that they have a narrow range of rotational drive speeds providing efficient operation.
Thus, it can be seen that a need exists for an aeration impeller capable of operating efficiently over a broader range of rotational drive speeds than is permitted by known impeller configurations. It is to the provision of such a device that the present invention is primarily directed.